The drilling of an oil well generally involves drilling the borehole in successive stages with each stage having a borehole diameter which is less than the borehole diameter of the preceding stage As each of the upper stages of the borehole is drilled, a string of tubular casing pipe is inserted into the borehole, extending from the earth's surface to a location adjacent the current bottom of the borehole. This string of casing pipe is generally cemented in place The column of cement, in the annulus formed by outer surface of the casing pipe and the borehole wall, supports the casing pipe and at least substantially prevents fluid migration along the annulus. Then a smaller diameter drill is lowered through the casing and the next stage of the borehole is drilled. Eventually extending the new casing all the way from the earth surface becomes undesirable, and then the newest borehole stage is cased by lowering a string of liner pipe into the borehole and hanging this liner from the lower end of the lowermost string of pipe already in the borehole, followed by cementing the liner in place. The drilling, lining and cementing operations can continue until the desired depth is achieved.
To position and cement a liner in a string of casing, the liner is made up with the usual bottom hole equipment which includes a casing shoe, float collar and plug catchers and is connected up to the desired length. At the top of the liner is a liner hanger which is an assembly having slip elements which are normally retracted while going into the borehole and which are released downhole when setting of the liner hanger is desired. The liner hanger is lowered into the borehole by a setting tool which attaches to the liner hanger and a string of pipe attached to the setting tool. At the desired location where the casing shoe is preferably located above the bottom of the open borehole, the liner hanger is set in the next above casing by actuating the setting tool to set the slips on the liner hanger. Upon setting the liner hanger, the weight of the liner is suspended by the liner hanger on the next above casing. The setting tool is released and the liner hanger is then cemented by pumping cement through the string of pipe and through the liner and into the annulus between the liner and borehole After the cement is set up, any remaining cement in the liner can be removed by drilling through the liner and destructible cement equipment at the lower end of the liner. When the open borehole reaches the projected well depth and traverses the formations to be completed, the liner includes a liner hanger and oftentimes a polished bore receptacle (PBR). The polished bore receptacle attaches either above or below the liner hanger and provides a bore to receive a sealing member on a production tubing string.
The production string of tubing, which has a sealing element adapted to be slidingly and sealingly received in the polished bore receptacle, extends from the earth's surface. When the well is completed, fluids from the earth formation being produced flow through the tubing string to the production equipment at the earth's surface. The sealing element on the string of tubing is subjected to downhole hydraulic pressure forces, and the tubing string is subjected to expansion and contraction forces due to changes in the temperature of the fluid in the tubing string. The purpose of a sliding seal on a string of tubing in a polished bore receptacle generally is to permit movement of the seal and the string of tubing relative to the polished bore receptacle.
When the producing formation is at high pressure conditions, it is possible for high pressure fluid to seep through the annular sleeve of concrete which exists between the outside of the liner and the surrounding earth formation. Such seepage can rise and enter the annular gap between the overlap of the top of the liner in the producing formation and the bottom of the next higher cemented string of pipe. In the absence of any sealing mechanism for the annular gap, the high pressure fluid can fill the annulus between the production tubing and the casing. The production tubing can have a higher pressure rating for a lower cost than the casing pipe because of the relatively small diameter of the production tubing compared to the diameter of the casing pipe. Accordingly, it is generally desirable to reduce costs by using casing pipe with a substantially lower pressure rating than the production tubing Thus, leakage of the high pressure fluid into the annulus between the production tubing and the casing can cause damage to the lower pressure rated casing.
One solution for the leakage problem is to install a packer to seal the annular gap between the liner and the casing, in combination with a second polished bore receptacle (PBR) positioned within the packer to seal off the annulus between the production tubing and the liner. Each PBR can be as much as thirty feet long and is very expensive due to the manufacturing requirements for its polished interior surface. In many downhole completions using a permanent packer, the tailpipe is run as part of the packer assembly and becomes a permanent part of the well, unless the permanent packer is also removed. The presence of the tailpipe in the wellbore restricts access to the liner because of the small size of the tailpipe. Some downhole completions utilize a landed seal assembly, but this requires two trips to retrieve.
Accordingly, it is an object of the present invention to provide new and improved method and apparatus for isolating a liner and providing a seal between production tubing and the liner. Another object of the invention is to permit the downhole production completion assembly to be disconnected from the liner isolation seal and be removed from the borehole in a single trip. A further object of the invention is to permit the removal of the production tubing from the borehole while maintaining a high pressure seal between the producing formation and the borehole casing above the producing formation. Other objects, aspects and advantages of the invention will be apparent from the following description, the drawings and the appended claims to the invention.